Recently, small wireless devices such as mobile phones have been prevailing rapidly, and there is a requirement for small and wideband antennas to be mounted on such wireless devices. An example of an antenna capable of meeting such a requirement is a monopole antenna.
The monopole antenna is an antenna including a radiating element connected with an internal conductor of a coaxial cable and a ground (also referred to as “bottom board”) connected with an external conductor of the coaxial cable. In particular, a monopole antenna including a short-circuit section which short-circuits a radiating element and a ground is called an inverted F antenna. Such a monopole antenna can reduce the entire length of a radiating element to approximately ¼ of an operating wavelength, and accordingly is advantageous in terms of downsizing compared to a dipole antenna operating at the same band (whose radiating element is required to have an entire length of approximately ½ of an operating wavelength).
Known examples of a technique for further downsizing the monopole antenna without limiting an operating band are described in, for example, Patent Literatures 1 and 2. Patent Literature 1 discloses an inverted F antenna in which a radiating element (element part) is turned back so as to be downsized. Patent Literature 2 discloses an inverted F antenna in which a ground (second conductor) is notched so as to reduce the area of a bottom board.